Summary

For nearly 50 years, the United States has led the world in the scientific exploration of space. U.S. spacecraft have circled Earth, landed on the Moon and Mars, orbited Jupiter and Saturn, and traveled beyond the orbit of Pluto and out of the ecliptic. These spacecraft have sent back to Earth images and data that have greatly expanded human knowledge, though many important questions remain unanswered.

Spacecraft require electrical energy. This energy must be available in the outer reaches of the solar system where sunlight is very faint. It must be available through lunar nights that last for 14 days, through long periods of dark and cold at the higher latitudes on Mars, and in high-radiation fields such as those around Jupiter. Radioisotope power systems (RPSs) are the only available power source that can operate unconstrained in these environments for the long periods of time needed to accomplish many missions, and plutonium-238 (238Pu) is the only practical isotope for fueling them. The success of historic missions such as Viking and Voyager, and more recent missions such as Cassini and New Horizons, clearly show that RPSs—and an assured supply of 238Pu—have been, are now, and will continue to be essential to the U.S. space science and exploration program.

Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs) are the only RPS currently available. MMRTGs convert the thermal energy that is released by the natural radioactive decay of 238Pu to electricity using thermocouples. This is a proven, highly reliable technology with no moving parts.

The Advanced Stirling Radioisotope Generator (ASRG) is a new type of RPS that is still being developed. An ASRG uses a Stirling engine (with moving parts) to convert thermal energy to electricity. Stirling engine converters are much more efficient than thermocouples. As a result, ASRGs produce more electricity than MMRTGs, even though they require only one-fourth as much 238Pu. It remains to be seen, however, when development of a flight-qualified ASRG will be completed.

THE PROBLEM

Plutonium-238 does not occur in nature. Unlike 239Pu, it is unsuitable for use in nuclear weapons. Plutonium-238 has been produced in quantity only for the purpose of fueling RPSs. In the past, the United States had an adequate supply of 238Pu, which was produced in facilities that existed to support the U.S. nuclear weapons program. The problem is that no 238Pu has been produced in the United States since the Department of Energy (DOE) shut down those facilities in the late 1980s. Since then, the U.S. space program has had to rely on the inventory of 238Pu that existed at that time, supplemented by the purchase of 238Pu from Russia. However, Russian facilities that produced 238Pu were also shut down many years ago, and the DOE will soon take delivery of its last shipment of 238Pu from Russia. The committee does not believe that there is any additional 238Pu (or any operational 238Pu production facilities) available anywhere in the world. The total amount of 238Pu available for NASA is fixed, and essentially all of it is already dedicated to support several pending missions—the Mars Science Laboratory, Discovery 12, the Outer Planets Flagship 1 (OPF 1), and (perhaps) a small number of additional missions with a very small demand for 238Pu. If the status quo persists, the United States will not be able to provide RPSs for any subsequent missions.

Reestablishing domestic production of 238Pu will be expensive; the cost will likely exceed $150 million. Previous proposals to make this investment have not been enacted, and cost seems to be the major impediment. However, regardless of why these proposals have been rejected, the day of reckoning has arrived. NASA is already making mission-limiting decisions based on the short supply of 238Pu. NASA is stretching out the pace of RPS-powered missions by eliminating RPSs as an option for some missions and delaying other missions that require RPSs until more 238Pu becomes available. Procuring 238Pu from Russia or other



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summary the prOBLeM For nearly 50 years, the United States has led the world in the scientific exploration of space. U.S. spacecraft have Plutonium-238 does not occur in nature. Unlike 239Pu, it circled Earth, landed on the Moon and Mars, orbited Jupiter is unsuitable for use in nuclear weapons. Plutonium-238 has and Saturn, and traveled beyond the orbit of Pluto and out of been produced in quantity only for the purpose of fueling the ecliptic. These spacecraft have sent back to Earth images RPSs. In the past, the United States had an adequate supply and data that have greatly expanded human knowledge, of 238Pu, which was produced in facilities that existed to though many important questions remain unanswered. support the U.S. nuclear weapons program. The problem is Spacecraft require electrical energy. This energy must that no 238Pu has been produced in the United States since be available in the outer reaches of the solar system where the Department of Energy (DOE) shut down those facilities sunlight is very faint. It must be available through lunar in the late 1980s. Since then, the U.S. space program has nights that last for 14 days, through long periods of dark and had to rely on the inventory of 238Pu that existed at that time, cold at the higher latitudes on Mars, and in high-radiation supplemented by the purchase of 238Pu from Russia. How- fields such as those around Jupiter. Radioisotope power ever, Russian facilities that produced 238Pu were also shut systems (RPSs) are the only available power source that can down many years ago, and the DOE will soon take delivery operate unconstrained in these environments for the long of its last shipment of 238Pu from Russia. The committee periods of time needed to accomplish many missions, and does not believe that there is any additional 238Pu (or any plutonium-238 (238Pu) is the only practical isotope for fuel- operational 238Pu production facilities) available anywhere in ing them. The success of historic missions such as Viking and the world. The total amount of 238Pu available for NASA is Voyager, and more recent missions such as Cassini and New fixed, and essentially all of it is already dedicated to support Horizons, clearly show that RPSs—and an assured supply of several pending missions—the Mars Science Laboratory, 238Pu—have been, are now, and will continue to be essential Discovery 12, the Outer Planets Flagship 1 (OPF 1), and to the U.S. space science and exploration program. (perhaps) a small number of additional missions with a very Multi-Mission Radioisotope Thermoelectric Generators small demand for 238Pu. If the status quo persists, the United (MMRTGs) are the only RPS currently available. MMRTGs States will not be able to provide RPSs for any subsequent convert the thermal energy that is released by the natural missions. radioactive decay of 238Pu to electricity using thermocouples. Reestablishing domestic production of 238Pu will be This is a proven, highly reliable technology with no moving expensive; the cost will likely exceed $150 million. Previous parts. proposals to make this investment have not been enacted, and The Advanced Stirling Radioisotope Generator (ASRG) cost seems to be the major impediment. However, regard- is a new type of RPS that is still being developed. An ASRG less of why these proposals have been rejected, the day of uses a Stirling engine (with moving parts) to convert thermal reckoning has arrived. NASA is already making mission- energy to electricity. Stirling engine converters are much limiting decisions based on the short supply of 238Pu. more efficient than thermocouples. As a result, ASRGs NASA is stretching out the pace of RPS-powered missions produce more electricity than MMRTGs, even though they by eliminating RPSs as an option for some missions and require only one-fourth as much 238Pu. It remains to be seen, delaying other missions that require RPSs until more 238Pu however, when development of a flight-qualified ASRG will becomes available. Procuring 238Pu from Russia or other be completed. 

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 RADIOISOTOPE POWER SYSTEMS foreign nations is not a viable option because of schedule and somewhere between the best-case curve and the status-quo national security considerations. Fortunately, there are two curve in Figure S.1, and it could easily be 20 kg or more over viable approaches for reestablishing production of 238Pu in the next 15 to 20 years. the United States. Both of these approaches would use exist- It has long been recognized that the United States would need to restart domestic production of 238Pu in order to ing reactors at DOE facilities at Idaho National Laboratory and Oak Ridge National Laboratory with minimal modifica- continue producing RPSs and to maintain U.S. leadership tion, but a large capital investment in processing facilities in the exploration of the solar system. The problem is that would still be needed. Nonetheless, these are the best options the United States has delayed taking action to the point that in terms of cost, schedule, and risk for producing 238Pu in the situation has become critical. Continued inaction will exacerbate the magnitude and the impact of future 238Pu time to minimize the disruption in NASA’s space science and exploration missions powered by RPSs. shortfalls, and it will force NASA to make additional, dif- ficult decisions that will reduce the science return of some missions and postpone or eliminate other missions until a iMMediate actiOn is required source of 238Pu is available. The schedule for reestablishing 238Pu production will On April 29, 2008, the NASA administrator sent a letter to the secretary of energy with an estimate of NASA’s future have to take into account many factors, such as construction demand for 238Pu.1 The committee has chosen to use this of DOE facilities, compliance with safety and environmen- letter as a conservative reference point for determining the tal procedures, and basic physics. This schedule cannot be future need for RPSs. However, the findings and recommen- easily or substantially accelerated, even if much larger appro- dations in this report are not contingent on any particular priations are made available in future years in an attempt to set of mission needs or launch dates. Rather, they are based overcome the effects of ongoing delays. The need is real, and on a conservative estimate of future needs based on various there is no substitute for immediate action. future mission scenarios. The estimate of future demand HIGH-PRIORITY RECOMMENDATION. Plutonium- for 238Pu (which is about 5 kg/year) is also consistent with 238 Production. The fiscal year 2010 federal budget should historic precedent. The orange line [hollow square data points] in Figure S.1 fund the Department of Energy (DOE) to reestablish produc- shows NASA’s cumulative future demand for 238Pu in a best- tion of 238Pu. case scenario (which is to say, a scenario in which NASA’s future RPS-mission set is limited to those missions listed • As soon as possible, the DOE and the Office of Man- in the NASA administrator’s letter of April 2008, the 238Pu agement and Budget should request—and Congress required by each mission is the smallest amount listed in should provide—adequate funds to produce 5 kg of 238Pu per year. that letter, and ASRGs are used to power OPF 1). The green line [solid square data points] shows NASA’s future demand • NASA should issue annual letters to the DOE defining the future demand for 238Pu. if the status quo persists (which is to say, if OPF 1 uses MMRTGs). Once the DOE is funded to reestablish production of deveLOpMent Of a fLight-readY advanced 238Pu, it will take about 8 years to begin full production of stirLing radiOisOtOpe generatOr 5 kg/year. The red and blue lines [triangular data points] in Figure S.1 show the range of future possibilities for 238Pu Advanced RPSs are required to support future space missions while making the most out of whatever 238Pu is balance (supply minus demand). A continuation of the status quo, with MMRTGs used for OPF 1 and no production of available. Until 2007, the RPS program was a technol- 238Pu, leads to the largest shortfall, and the balance curve ogy development effort. At that time, the focus shifted to drops off the bottom of the chart. The best-case scenario, development of a flight-ready ASRG, and that remains the which assumes that OPF 1 uses ASRGs and DOE receives current focus of the RPS program. The program received no funding in fiscal year (FY) 2010 to begin reestablishing its additional funds to support this new tasking, so funding for ability to produce 238Pu, yields the smallest shortfall (as several other important RPS technologies was eliminated, little as 4.4 kg). However, it seems unlikely that all of the and the budget for the remaining RPS technologies was cut. assumptions that are built into the best-case scenario will As a result, the RPS program is not well balanced. Indeed, come to pass. MMRTGs are still baselined for OPF 1, there balance is impossible given the current (FY 2009) budget and remains no clear path to fight qualification of ASRGs, and the focus on development of flight-ready ASRG technology. FY 2010 funding for 238Pu production remains more a hope However, the focus on ASRG development is well aligned than an expectation. Thus, the actual shortfall is likely to be with the central and more pressing issue that threatens the future of RPS-powered missions: the limited supply of 238Pu. The RPS program should continue to support NASA’s 1Letter from the NASA Administrator Michael D. Griffin to Secretary of mission requirements for RPSs while minimizing NASA’s Energy Samuel D. Bodman, April 29, 2008 (reprinted in Appendix C).

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 SUMMARY 120 Pu Balance = Supply − Demand 100 Pu demand (status quo) 80 K i log ram s o f P u -238 60 Pu demand (best case) 40 Pu balance 20 (best case) 0 Pu balance (status quo) T he Problem -20 -40 2008 2010 2012 2014 2016 2018 2020 2022 2024 2026 2028 Calendar Year Pu demand, status quo: OPF 1 uses MMRTGs Pu demand, best case: OPF 1 uses ASRGs Pu balance, status quo: OPF 1 uses MMRTGs, with no new Pu production Pu balance, best case: OPF 1 uses ASRGs, FY 2010 funding for Pu production FIGURE S.1 Potential 238Pu demand and net balance, 2008 through 2028. demand for 238Pu. NASA should continue to move the ASRG RPSs in general and ASRGs in particular would facilitate project forward, even though this has come at the expense of S-1 the acceptance of ASRGs as a viable option for deep-space other RPS technologies. missions and reduce the impact that the limited supply of 238Pu will have on NASA’s ability to complete important Demonstrating the reliability of ASRGs for a long-life mission is critical, but it has yet to be achieved. The next space missions. major milestones in the advancement of ASRGs are to RECOMMENDATION. Flight Readiness. The RPS pro- freeze the design of the ASRG, conduct system testing that verifies that all credible life-limiting mechanisms have been gram and mission planners should jointly develop a set identified and assessed, and demonstrate that ASRGs are of flight-readiness requirements for RPSs in general and ready for flight. In lieu of any formal guidance or require - Advanced Stirling Radioisotope Generators in particular, as ments concerning what constitutes flight readiness, ongoing well as a plan and a timetable for meeting the requirements. efforts to advance ASRG technology and demonstrate that RECOMMENDATION. Technology Plan. NASA should it is flight ready are being guided by experience gained from past programs and researchers’ best estimates about develop and implement a comprehensive RPS technology the needs and expectations of project managers for future plan that meets NASA’s mission requirements for RPSs while minimizing NASA’s demand for 238Pu. This plan missions. While this approach has enabled progress, the establishment of formal guidance for flight certification of should include, for example:

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 RADIOISOTOPE POWER SYSTEMS RECOMMENDATION. Multi-Mission RTGs. NASA • A prioritized set of program goals. • A prioritized list of technologies. and/or the Department of Energy should maintain the ability • A list of critical facilities and skills. to produce Multi-Mission Radioisotope Thermoelectric • A plan for documenting and archiving the knowledge Generators. base. HIGH-PRIORITY RECOMMENDATION. ASRG • A plan for maturing technology in key areas, such as Development. NASA and the Department of Energy (DOE) reliability, power, power degradation, electrical inter- faces between the RPS and the spacecraft, thermal should complete the development of the Advanced Stirling interfaces, and verification and validation. Radioisotope Generator (ASRG) with all deliberate speed, • A plan for assessing and mitigating technical and sched- with the goal of demonstrating that ASRGs are a viable ule risk.,., option for the Outer Planets Flagship 1 mission. As part of this effort, NASA and the DOE should put final-design ASRGs on life test as soon as possible (to demonstrate reli- ability on the ground) and pursue an early opportunity for operating an ASRG in space (e.g., on Discovery 12).